fix(quic): tier-local round-robin so priority survives every drain
The previous priority-then-round-robin shape applied the rotating start-index globally over the sorted list, so the cross-tier order flipped on alternate drains: drain N had high-priority first, drain N+1 advanced streamRoundRobinStart and had low-priority first. The priority hint was silently defeated under any sustained traffic. Replace it with strict priority across tiers + round-robin only within each same-priority tier. Higher tiers always drain ahead of lower ones; same-priority peers continue to take turns via the existing rotating start. Default-priority callers see no behaviour change (single tier, identical rotation semantics). Tighten the test: drain twice and assert the higher-priority stream emits first on BOTH drains — the regression case that the single- drain version of the test missed. Add a regression guard for the same-priority round-robin so a future refactor can't silently serialise on the first stream. https://claude.ai/code/session_01KWdr4RjVvyYZfEuPVaQfUa
This commit is contained in:
Claude
+64
-48
@@ -410,58 +410,74 @@ private fun buildApplicationPacket(
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// insertion-ordered and stays in sync with the streams map.
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val streamsView = conn.streamsListLocked()
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if (streamsView.isNotEmpty()) {
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// Priority-then-round-robin: stable sortedByDescending preserves
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// insertion order within a tier, so same-priority streams keep
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// the rotating start-index round-robin behaviour. Higher-priority
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// streams (e.g. moq-lite newer-sequence group streams) drain
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// first under congestion. Default priority is 0; if every stream
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// is at the default, iteration order matches pre-priority code.
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// Cost: O(N log N) per drain pass and one transient list
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// allocation. N is small (1–10 in the moq-lite audio path); if
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// it ever grows enough to matter, switch to an indirect index
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// sort or maintain an incrementally-sorted view on setPriority.
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// Strict priority across tiers, round-robin within each tier.
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// Higher-priority streams (e.g. moq-lite newer-sequence group
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// streams) ALWAYS drain ahead of lower-priority ones; the
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// rotating start-index only rotates among same-priority peers.
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// This is the spec-aligned shape — applying the rotation
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// globally over the sorted list would flip cross-tier order on
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// alternating drains, defeating the priority hint entirely.
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//
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// Default priority is 0; if every stream is at the default, all
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// streams form a single tier and iteration order matches the
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// pre-priority round-robin behaviour exactly.
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//
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// Cost: O(N log N) per drain pass plus one transient sorted
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// list. N is small (1–10 in the moq-lite audio path); if it
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// ever grows enough to matter, switch to an indirect index sort
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// or maintain an incrementally-sorted view on setPriority.
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val sorted =
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if (streamsView.size > 1) streamsView.sortedByDescending { it.priority } else streamsView
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val start = conn.streamRoundRobinStart % sorted.size
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for (i in sorted.indices) {
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if (packetBudget <= 64) break
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val stream = sorted[(start + i) % sorted.size]
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val streamRemaining = (stream.sendCredit - stream.send.sentOffset).coerceAtLeast(0L)
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// Skip if both stream and connection have no credit; FIN-only
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// (zero-byte) chunks may still go through because they don't
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// consume credit.
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if (streamRemaining <= 0L && connBudget <= 0L && !stream.send.finPending) continue
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val effectiveCap = minOf(streamRemaining, connBudget)
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val maxBytes =
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minOf(packetBudget - 32, effectiveCap.coerceAtMost(Int.MAX_VALUE.toLong()).toInt())
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val chunk = stream.send.takeChunk(maxBytes = maxBytes) ?: continue
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if (chunk.data.isNotEmpty() || chunk.fin) {
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frames +=
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StreamFrame(
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streamId = stream.streamId,
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offset = chunk.offset,
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data = chunk.data,
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fin = chunk.fin,
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explicitLength = true,
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)
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// Step C of the deferred-follow-ups pass: track this
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// STREAM emission so RFC 9002 retransmit can re-queue
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// the byte range on loss. SendBuffer.markLost (commit B)
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// moves the range from in-flight back to the retransmit
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// queue, and the next takeChunk replays it.
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tokens +=
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RecoveryToken.Stream(
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streamId = stream.streamId,
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offset = chunk.offset,
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length = chunk.data.size.toLong(),
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fin = chunk.fin,
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)
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packetBudget -= chunk.data.size + 32
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connBudget -= chunk.data.size
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conn.sendConnectionFlowConsumed += chunk.data.size
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val rotation = conn.streamRoundRobinStart
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var tierStart = 0
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outer@ while (tierStart < sorted.size) {
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// Walk the contiguous run of same-priority streams.
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val tierPriority = sorted[tierStart].priority
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var tierEnd = tierStart + 1
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while (tierEnd < sorted.size && sorted[tierEnd].priority == tierPriority) tierEnd++
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val tierSize = tierEnd - tierStart
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val tierRotation = if (tierSize > 1) rotation % tierSize else 0
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for (k in 0 until tierSize) {
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if (packetBudget <= 64) break@outer
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val stream = sorted[tierStart + ((tierRotation + k) % tierSize)]
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val streamRemaining = (stream.sendCredit - stream.send.sentOffset).coerceAtLeast(0L)
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// Skip if both stream and connection have no credit; FIN-only
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// (zero-byte) chunks may still go through because they don't
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// consume credit.
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if (streamRemaining <= 0L && connBudget <= 0L && !stream.send.finPending) continue
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val effectiveCap = minOf(streamRemaining, connBudget)
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val maxBytes =
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minOf(packetBudget - 32, effectiveCap.coerceAtMost(Int.MAX_VALUE.toLong()).toInt())
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val chunk = stream.send.takeChunk(maxBytes = maxBytes) ?: continue
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if (chunk.data.isNotEmpty() || chunk.fin) {
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frames +=
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StreamFrame(
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streamId = stream.streamId,
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offset = chunk.offset,
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data = chunk.data,
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fin = chunk.fin,
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explicitLength = true,
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)
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// Step C of the deferred-follow-ups pass: track this
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// STREAM emission so RFC 9002 retransmit can re-queue
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// the byte range on loss. SendBuffer.markLost (commit B)
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// moves the range from in-flight back to the retransmit
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// queue, and the next takeChunk replays it.
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tokens +=
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RecoveryToken.Stream(
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streamId = stream.streamId,
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offset = chunk.offset,
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length = chunk.data.size.toLong(),
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fin = chunk.fin,
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)
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packetBudget -= chunk.data.size + 32
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connBudget -= chunk.data.size
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conn.sendConnectionFlowConsumed += chunk.data.size
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}
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}
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tierStart = tierEnd
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}
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conn.streamRoundRobinStart = (start + 1) % streamsView.size
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conn.streamRoundRobinStart = (rotation + 1) % streamsView.size
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}
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if (frames.isEmpty()) return null
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+68
-32
@@ -112,45 +112,81 @@ class QuicConnectionWriterTest {
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@Test
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fun writer_drains_higher_priority_streams_before_lower_priority() {
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// T11.3 follow-up: the writer's drain loop must iterate streams
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// in descending priority order so moq-lite group streams with a
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// higher sequence number drain ahead of older ones under
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// congestion. Pre-fix this test, the writer iterated in stable
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// round-robin order regardless of priority — so a backlog of
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// retransmits on an older group could starve the listener of
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// fresh frames. We pin the load-bearing invariant by inspecting
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// StreamFrame order in a single emitted packet: low-priority
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// stream is opened FIRST (so insertion order would normally win
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// round-robin), but the high-priority stream's bytes must land
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// earlier in the packet.
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// T11.3 follow-up: priority must dominate iteration order on
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// EVERY drain, not just the first. The naive "sort by priority
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// then apply the existing rotating start globally" shape looks
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// right at a glance but the rotating start advances on every
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// drain, so cross-tier ordering flips on alternate drains and
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// the priority hint is silently defeated. The correct shape is
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// strict priority across tiers + round-robin only within a
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// tier, which we pin here by draining TWICE and asserting the
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// higher-priority stream's StreamFrame lands first in BOTH
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// packets. Low-priority stream is opened FIRST so insertion-
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// order can't accidentally pass for priority ordering.
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runBlocking {
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val (client, pipe) = connectedClient()
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val low = client.openBidiStream()
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val high = client.openBidiStream()
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low.priority = 0
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high.priority = 10
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// Distinct payloads small enough to coexist in one packet.
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val lowPayload = ByteArray(200) { 0xAA.toByte() }
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val highPayload = ByteArray(200) { 0xBB.toByte() }
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low.send.enqueue(lowPayload)
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high.send.enqueue(highPayload)
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val datagram = drainOutbound(client, nowMillis = 0L)
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assertNotNull(datagram, "drain must emit a packet")
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val frames = pipe.decryptClientApplicationFrames(datagram)
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assertNotNull(frames, "decrypt must succeed at the application level")
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val streamFrames = frames.filterIsInstance<StreamFrame>()
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assertEquals(
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2,
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streamFrames.size,
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"expected one StreamFrame per stream in this drain, got $streamFrames",
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)
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assertEquals(
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high.streamId,
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streamFrames[0].streamId,
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"higher-priority stream must drain first; saw ${streamFrames.map { it.streamId }}",
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)
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assertEquals(low.streamId, streamFrames[1].streamId)
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repeat(2) { round ->
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low.send.enqueue(ByteArray(200) { 0xAA.toByte() })
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high.send.enqueue(ByteArray(200) { 0xBB.toByte() })
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val datagram = drainOutbound(client, nowMillis = 0L)
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assertNotNull(datagram, "drain on round $round must emit a packet")
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val frames = pipe.decryptClientApplicationFrames(datagram)
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assertNotNull(frames, "decrypt must succeed on round $round")
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val streamFrames = frames.filterIsInstance<StreamFrame>()
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assertEquals(
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2,
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streamFrames.size,
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"expected one StreamFrame per stream on round $round, got $streamFrames",
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)
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assertEquals(
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high.streamId,
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streamFrames[0].streamId,
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"higher-priority stream must drain first on round $round; " +
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"saw ${streamFrames.map { it.streamId }}",
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)
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assertEquals(low.streamId, streamFrames[1].streamId, "low second on round $round")
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}
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}
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}
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@Test
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fun writer_round_robins_within_a_priority_tier() {
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// Regression guard for the tier-local round-robin: same-priority
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// streams must still rotate so an early-opened stream doesn't
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// monopolise a packet's stream-frame slot indefinitely. We open
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// three streams at the default (0) priority and verify the
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// rotating start advances by one per drain, matching the
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// pre-priority behaviour.
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runBlocking {
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val (client, pipe) = connectedClient()
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val a = client.openBidiStream()
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val b = client.openBidiStream()
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val c = client.openBidiStream()
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// All default priority — single tier, three streams.
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val expectedRotation =
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listOf(
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listOf(a.streamId, b.streamId, c.streamId),
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listOf(b.streamId, c.streamId, a.streamId),
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listOf(c.streamId, a.streamId, b.streamId),
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)
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for ((round, expected) in expectedRotation.withIndex()) {
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a.send.enqueue(ByteArray(64) { 0xA1.toByte() })
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b.send.enqueue(ByteArray(64) { 0xB2.toByte() })
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c.send.enqueue(ByteArray(64) { 0xC3.toByte() })
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val datagram = drainOutbound(client, nowMillis = 0L)
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assertNotNull(datagram, "drain $round must emit a packet")
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val frames = pipe.decryptClientApplicationFrames(datagram)
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assertNotNull(frames, "decrypt must succeed on round $round")
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val ids = frames.filterIsInstance<StreamFrame>().map { it.streamId }
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assertEquals(expected, ids, "round $round round-robin order")
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}
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}
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}
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